As described in PTL 1 for example, conventionally known is that: an exhaust passage through which an exhaust gas is supplied to a turbine is divided into two independent passages; an exhaust cut valve is disposed on one of the two passages and opens and closes the passage; and the exhaust cut valve is switched between an open state and a closed state to switch a flow velocity of the exhaust gas between two velocities that are high and low velocities in front of the turbine in the exhaust passage. To be specific, in an engine operation range where the amount of exhaust gas is small, the exhaust cut valve is set to the closed state, and the exhaust gas is supplied to the turbine through only one passage. With this, the flow velocity of the exhaust gas is made higher than the flow velocity of the exhaust gas of the exhaust passage that is not divided into two passages, and thus, a required supercharging effect is secured. In an operation range where the amount of exhaust gas is large, the exhaust cut valve is opened, and the exhaust gas is supplied to the turbine through all the passages. With this, a decrease in an engine output due to an increase in exhaust pressure is prevented.
PTL 1 further includes: an exhaust bypass passage through which the exhaust gas flows to bypass the turbine; and an exhaust bypass valve (wastegate valve) disposed on the exhaust bypass passage. In each of an open operation range where the exhaust cut valve is in the open state and a closed operation range where the exhaust cut valve is in the closed state, feedback control of an opening degree of the exhaust bypass valve is performed such that actual supercharging pressure becomes preset target supercharging pressure in accordance with engine operation states.
The above turbocharged engine cannot perform the above-described feedback control when an open/close function of the exhaust cut valve is in failure. In PTL 1, when the supercharging pressure is not more than a first set supercharging pressure value in the closed operation range, it is determined that an open failure in which the exhaust cut valve is stuck in the open state occurs. On the other hand, when the supercharging pressure is not more than a second set supercharging pressure value in the open operation range, it is determined that a closed failure in which the exhaust cut valve is stuck in the closed state occurs. At the time of the open failure, the exhaust bypass valve is corrected in a closing direction as compared to normal supercharging pressure control. At the time of the closed failure, the exhaust bypass valve is corrected in an opening direction as compared to the normal supercharging pressure control.